The H-R Diagram

How light from the stars can tell us (almost) everything about them!

How Hot Is It?

• Photometry: measuring a star’s temperature by studying its color

• What color is the hottest? The coldest?

• Hottest: Blue/Violet

• Coldest: Red

NO!!! YES!!!

Stellar Spectroscopy

• Can identify the chemical composition ANDthe temperature of stars!

• How?

• Excitation energy of electrons in their shells –Quantum Mechanics

Spectral Classification

• Using spectroscopy to relate directly to stellar surface temperature.

• Fixed by Annie Jump Cannon

• Classifies stars by temperature from hottest to coolest

• Directly related to a star’s color

O B A F G K M

40,000 K

2,000 K

O B A F G K M• There are sub-

ranges

0 – 10

0 = hottest

10 = coolest

For example:

B4 is a hotter star than B8

F9 is hotter than G2

• Our Sun?

A G2 star

Patterns Emerge When Luminosity (power) is plotted against Temperature

1911 – Ejnar Hertzsprung 1913 – Henry Russell

Stars don’t have random temperatures and colors! They are directly related!

Luminosity Class

• The measure of Watts produced by the star

• Also able to be seen by spectroscopy

• White Dwarfs usually not included (no fusion)

The H-R Diagram plots FOUR aspects of stars

The 2 Pairs are Directly Related:

Spectral Class & Temperature

Luminosity & Magnitude

Remember Stellar Lifecycles?

• Interactive HR Diagram

HR Diagram Project• Create your own HR Diagram

using people or objects!

• Follow a theme: a family, a TV show, movie, or book characters, etc.

• Can be digital or on paper

• Must demonstrate four different stages of star life:

1. Protostar

2. Main Sequence

3. Giant/Supergiant

4. White Dwarf/NS/Black Hole